The present invention relates to an optical fiber transmission system using soliton signals and wavelength division multiplexing, and to a method of transmission in such a system, the system and the method making it possible to reduce the effects of collision-induced jitter. It also relates to a method and a device for interchanging wavelengths in an optical fiber transmission system using soliton signals and wavelength division multiplexing.
It is known that soliton impulses or xe2x80x9csolitonsxe2x80x9d can be transmitted in that portion of an optical fiber which has abnormal dispersion. Soliton signals are pulse signals having waveform of the sech2 type. With that type of pulse, the non-linearity in the corresponding portion of the fiber compensates for the dispersion of the optical signal. Soliton transmission is modelled in known manner by the non-linear Schrodinger equation.
Transmission of such pulses is limited by various effects, such as jitter induced by the solitons interacting with the noise present in the transmission system, as described, for example, in the article by J. P. Gordon and H. A. Haus, Optical Letters, vol. 11, No. 10, pages 665-667. That effect, known as the xe2x80x9cGordon-Haus effectxe2x80x9d or as xe2x80x9cGordon-Haus jitterxe2x80x9d, sets a theoretical limit on the quality or on the data rate of soliton transmission.
In order to overcome those limits, it is possible to perform synchronous modulation on the soliton signals, by means of semiconductor modulators. Systems using sliding guiding filters have also been proposed, making it possible to control the jitter of the transmitted solitons, e.g. as described in EP-A-0 576 208. In order to regenerate the signal in-line, it has also been proposed to use the Kerr effect in synchronous amplitude or phase modulators, or to use saturable absorbents.
In addition, to increase the data rate of optical fiber transmission systems using soliton signals, it has also been proposed to use wavelength division multiplexing (WDM). In which case, it is considered to be advantageous to use sliding guiding filters of the Fabry Perot type, which filters are entirely compatible with wavelength division multiplexed signals. However, the use of synchronous modulators or of saturable absorbants for regenerating wavelength division multiplexed soliton signals is problematic because of the different group velocities between the signals of the various channels.
An article by E. Desurvire, O. Leclerc, and O. Audouin, Optics Letters, vol. 21, No. 14, pages 1026-1028 describes a wavelength allocation scheme which is compatible with the use of synchronous modulators. That article proposes allocating wavelengths to the various channels of the multiplex in a manner such that, for given intervals ZR between the repeaters, the signals of the various channels, or more exactly the bit times of the various channels of the multiplex, are substantially synchronized on arriving at the repeaters. Synchronous modulation can thus be performed in-line on all of the channels, at given intervals, by means of discrete synchronous modulators. That technique for allocating the wavelengths of the multiplex is also described in French Patent Application 96 00732 of Jan. 23, 1996 in the name of Alcatel Submarine Networks. The article proposes selecting a sub-group of channels which are synchronous not only at intervals ZR, but also at intervals that are sub-multiples of ZR. Other aspects of that technique for allocating wavelengths are described in an article by O. Leclerc, E. Desurvire and O. Audouin entitled xe2x80x9cSynchronous WDM Soliton Regeneration: Toward 80-160 Gbit/s Transoceanic Systemsxe2x80x9d, Optical Fiber Technology, 3, pages 97-116 (1997), and in an article by E. Desurvire et al entitled xe2x80x9cTransoceanic Regenerated Soliton Systems: Designs for over 100 Gbit/s Capacitiesxe2x80x9d, Suboptic ""97, pages 438-447.
An article by L. F. Mollenauer, S. G. Evangelides, and P. J. Gordon entitled xe2x80x9cWavelength Division Multiplexing with Solitons in Ultra Long Distance Transmissions using Lumped Amplifiersxe2x80x9d, Journal of Lightwave Technology, vol 9, No. 3, pages 362-367 (1991) describes the problem of collisions between solitons in wavelength division multiplexing systems, and emphasizes in particular the variations in propagation velocity induced by such collisions. At the output of the transmission system, such variations can induce unacceptable jitter on the solitons. That article explains that variations in the chromatic dispersion of the fiber along the transmission path can compensate for the effects of collisions. It is therefore proposed to use segments of different dispersion to compensate for the effects of collisions on the propagation velocity of the solitons, in a transmission system in which the distance between amplifiers is short compared with the length of the collisions.
That solution is difficult to apply on an industrial scale because of the constraints on managing the fibers, and because of the short spacing between the amplifiers. In addition, it is not applicable to WDM transmission systems that use wavelength allocation schemes of the type mentioned above, insofar as the variations in the dispersion of the fiber disturb the bit time synchronism at the synchronous regenerators.
An article by A. Hasegawa, S. Kumar, and Y. Kodoma entitled xe2x80x9cReduction of Collision-Induced Time-Jitter in Dispersion-Managed Soliton Transmission Systemsxe2x80x9d, Optics Letters, vol. 21, No. 1, January 1996, pages 39-41 proposes a scheme for managing in-fiber dispersion, which scheme makes it possible to increase the distance between amplifiers. That solution relies on a stepped dispersion profile in the fiber, which profile is as close as possible to an exponential ideal profile. That solution cannot be implemented industrially, and using it with the above-mentioned frequency allocation scheme is problematic.
An object of the invention is to provide a solution to the problem of managing collisions between solitons in a transmission system using wavelength division multiplexing, which solution avoids the need to cause the dispersion profile of the transmission fiber to vary along the transmission system. The invention is particularly advantageous in WDM transmission systems in which the wavelengths are chosen in a manner such that the bit times of the various channels are synchronous at given intervals. The invention proposes such a solution that is simple, and that limits time jitter due to collisions between solitons.
More precisely, the invention provides an optical fiber transmission system using wavelength division multiplexing and soliton signals, said system including, at least once, interchange means for interchanging the wavelengths of at least two channels, so as to change the sign of the residual frequency variation induced by asymmetrical collisions on the signals of said channels.
In an embodiment, the wavelengths (xcex1 to xcexn) of the various channels of the multiplex constitute a set comprising a bottom half and a top half, and the interchange means interchange the wavelengths of at least two channels so that the signals transmitted on a wavelength from one of said halves are transmitted on a wavelength from the other of said halves.
In which case, it is possible for the interchange means to perform wavelength interchange in a manner such that the signals transmitted on a wavelength xcexi from one of said halves are transmitted on a wavelength xcexnxe2x88x92i+1 from the other of said halves, where i is an integer taking at least one value in the range 1 to n.
Advantageously, the interchange means further interchange the wavelengths such that the signals transmitted on a wavelength xcexnxe2x88x92i+1 from one of said halves are transmitted on a wavelength xcexi from the other of said halves.
The integer i may take all of the possible values in the range 1 to n.
In an embodiment, the transmission system includes single interchange means disposed substantially in the middle of the transmission system.
In another embodiment, the system includes a plurality of interchange means spaced apart along the transmission system.
The invention also provides a method of transmitting soliton signals using wavelength division multiplexing, said method including at least one wavelength interchange step in which the wavelengths of at least two channels are interchanged, so as to change the sign of the residual frequency variation induced by asymmetrical collisions on the signals of said channels.
Preferably, the wavelengths (xcex1 to xcexn) of the various channels of the multiplex constitute a set comprising a bottom half and a top half, and the channel-interchange step takes place such that the signals transmitted on a wavelength from one of said halves are transmitted on a wavelength from the other of said halves.
The wavelength interchange advantageously takes place in a manner such that the signals transmitted on a wavelength xcexi from one of said halves are transmitted on a wavelength xcexnxe2x88x92i+1 from the other of said halves, where i is an integer taking at least one value in the range 1 to n.
The interchange step may further comprise interchanging the wavelengths in a manner such that the signals transmitted on a wavelength xcexnxe2x88x92i+1 from one of said halves are transmitted on a wavelength xcexi from the other of said halves.
Advantageously, the integer i takes all of the possible values in the range 1 to n.
The method may comprise a single interchange step which takes place after substantially one half of the propagation time.
The method may also comprise a plurality of interchange steps.
The invention further provides the use of such a method to reduce collision-induced jitter.
The invention also provides a wavelength interchange device, comprising:
demultiplexer means for demultiplexing a wavelength division multiplex of soliton signals, which means deliver a plurality of channels at wavelengths (xcex1 to xcexn), said wavelengths constituting a set formed by a bottom half and by a top half;
at least two wavelength converters for interchanging the wavelength of a channel chosen from one of said halves and the wavelength of a channel chosen from the other of said halves; and
multiplexer means for re-multiplexing the channels.
In an embodiment, the converters perform the wavelength interchange in a manner such that the signals received on a wavelength xcexi from one of said halves are transmitted on a wavelength xcexnxe2x88x92i+1 from the other of said halves, and vice versa, where i is an integer taking at least one value in the range 1 to n.
Preferably, the device includes 2Int(n/2) converters, where n is the number of channels.
The invention further provides a method of interchanging wavelengths in a wavelength division multiplex of soliton signals, the wavelengths (xcex1 to xcexn) constituting a set formed by a bottom half and by a top half, said method comprising:
a step of demultiplexing the channels of the multiplex;
a step of interchanging the wavelengths of at least one channel chosen from one of said halves and of at least one channel chosen from the other of said halves; and
a step of re-multiplexing the channels.
The interchange step advantageously comprises interchanging a wavelength xcexi from one of said halves and a wavelength xcexnxe2x88x92i+1 from the other of said halves, where i is an integer taking at least one value in the range 1 to n.
Preferably, the integer i takes all of the values in the range 1 to Int(n/2), where n is the number of channels.
The invention further provides a device for interchanging wavelengths within a wavelength division multiplex of soliton signals, the wavelengths (xcex1 to xcexn) of the multiplex constituting a set formed by a bottom half and by a top half, said device comprising:
means for separating the multiplex into first and second signals, the first signal comprising the channels whose wavelengths belong to one half, and the second signal comprising the channels whose wavelengths belong to the other half;
first conversion means for converting the wavelengths of the channels of the first signal into wavelengths of said other half;
second conversion means for converting the wavelengths of the channels of the second signal into wavelengths of said one half; and
combination means for combining the signals delivered by said first and second conversion means.
Advantageously, said first and second conversion means comprise four-wave mixers for conjugating the signals about a middle frequency (xcex1+xcexn)/2 midway between the end frequencies of the multiplex.
In which case, said first conversion means further comprise filters for filtering wavelengths other than those of said other half and said second conversion means further comprise filters for filtering wavelengths other than those of said one half, or vice versa.
Finally, the invention provides a method of interchanging wavelengths within a wavelength division multiplex of soliton signals, the wavelengths (xcex1 to xcexn) of the multiplex constituting a set formed by a bottom half and by a top half, said method comprising:
a step of separating the multiplex into first and second signals, the first signal comprising the channels whose wavelengths belong to one half, and the second signal comprising the channels whose wavelengths belong to the other half;
a step of converting the wavelengths of the channels of the first signal into wavelengths of said other half;
a step of converting the wavelengths of the channels of the second signal into wavelengths of said one half; and
a step of combining the first and second signals as converted.
Preferably, the conversion step comprises conversion by four-wave mixing to conjugate the signals about a middle frequency (xcex1+xcexn)/2 midway between the end frequencies of the multiplex.
In which case, the conversion step advantageously further comprises a filtering step after the four-wave mixing step.